“Snake” and “fish” robots are usually constituted by a series of independent modules separated by two articulated and motorized platforms—see the paper by C. Wright et al “Design and Architecture of the Unified Modular Snake Robot”, 2012 International Conference on Robotics and Automation (ICRA 2012), pages 4347-54. Each module has two degrees of freedom and bends in two perpendicular planes according to the degrees of freedom defined by a universal joint that connects the two platforms. This architecture does not allow any twisting movement. If necessary, a twisting degree of freedom is provided by a dedicated mobile element, which adds sophistication. Such a structure is used to actuate swimming and crawling drones and also in some industrial robots, e.g. painting robots in the industry field.
An alternative solution is based on the use of series of Stewart's platforms, each of said platforms having 6 degrees of freedom allowing bending, twisting and translation. However, the translations may not be needed in many applications. In these cases, the availability of a translational degree of freedom becomes a drawback. A further disadvantage of these systems is that the actuators of each platform must support the weight of the other platforms of the series and their load. In addition, their weight carrying capacity is low due to their cantilever structure: the stress load on the actuators of the first platform is very large and the structure tends to sag under the weight of the total load.
Recently, pneumatic systems have been proposed to implement trunk robots having the same kinematics (bending without twisting) as snake/fish robots. These machines are inherently compliant, a property which is not acceptable in some applications. See the paper by M. Rofl and J. Steil “Constant curvature continuum kinematics as fast approximate model for the Bionic Handling Assistant” IEEE/RSJ Int. Conf. Intelligent Robots and Systems (IROS), Vilamoura, Portugal: 3440-3446.